1. Field of the Invention
The present invention relates to a method and apparatus for measuring a halogen ion concentration and, more particularly, to a method and apparatus for measuring a halogen ion concentration using a halogen ion-selective electrode.
2. Description of the Prior Art
An ion-selective electrode has a linear relationship between its electromotive force E and a logarithm for concentration of specific ions in a solution. Thus, when an electromotive force of a solution, which contains specific ions at an unknown concentration, is measured, the concentration of the specific ions can be determined. In recent years, such as ion-selective electrode has been used in medical quantitative analysis of ions (e.g., sodium ions, potassium ions, chloride ions, and the like, particularly those contained in blood).
For chloride ion-selective electrodes, an electrode having on its surface a polyvinyl chloride film, in which chloride ion-selective material comprising a quanternary ammonium salt is dispersed, and an electrode having on its surface an ion-selective solid film, which is formed by compression molding a silver sulfide and silver chloride compound, have been developed and widely used in biochemical analyzers.
However, an organic material such as protein becomes easily attached to electrodes of the former constructions, or the electrodes are easily interfered with by other negative ions. Attachment of protein on such electrodes causes a serious problem: When serum protein becomes attached to an electrode surface during serum analysis, the sensitivity of the electrode is degraded, and response time is considerably delayed, thus shortening the electrode's working life.
In an electrode of the second construction, a mixing ratio of Ag.sub.2 S and AgCl must be accurately determined, and the electrode must be compression molded under high pressure, therefore, it is difficult to form a compact electrode, or an electrode in a desired shape, resulting in limited manufacture thereof. Furthermore, in order to simultaneously analyze several types of electrolytes in blood, a flow-through type ion sensor which integrates a chlorine ion-selective electrode, another ion-selective electrode, and an ion concentration measurement cell is preferably used. Particularly in the latter electrode, since it is difficult to mold it into a desired shape, such an electrode cannot be installed in the flow-through type ion sensor.
On the contrary, a silver/silver halide electrode (e.g., a silver/silver chloride electrode) is relatively free from the above problems, and has been receiving a great deal of attention for medical applications. However, in general, dissolution of silver ions from the silver/silver halide electrode proceeds until a saturated silver halide layer is formed around the electrode, and thus the electrode potential deviates correspondingly. As a result, the silver/silver halide electrode exhibits poor responsivity to halogen ions to be measured, namely exhibits delayed response time. This problem becomes particularly significant in a biochemical analysis wherein very small changes of halogen ion concentrations must be detected accurately. For example, blood serum usually contains chloride ions at a concentration of about 110 mEq. The concentration mentioned generally deviates within a range of +10 mEq in a normal serum. This deviation corresponds to electrode potential change of only .+-.2-.+-.3 mV. Potential change until a saturated silver chloride is formed around the electrode by elution of silver ions from the electrode is too large to be neglected against the above-noted potential change corresponding to the changes of the halogen ion concentration, and causes the delayed response.